The present invention generally relates to testing integrated circuits, and more particularly to a method and system capable of detecting potential reliability failures of memory chips.
“Bridging effect” is a term describing the formation of an undesired electrical connection between adjacent conducting lines in an integrated circuit. One cause of the bridging effect is metal migration induced by a voltage difference between the adjacent lines. Such undesired connection often causes short circuits and system failures to the integrated circuit. As wire space of integrated circuits becomes narrower, the metal migration becomes easier and the bridging effect becomes more troublesome. For embodiment, a memory chip has a plurality of closely located word lines and bit lines defining addresses of memory cells. Conventionally, those word lines are composed of poly-silicon lines strapped with metal lines in order to reduce the RC effect. The strapped metal lines are particularly susceptible to the bridging effect because the metal migration may easily cross their narrow spacing. This may results in an undesired connection formed between the metal lines. Such undesired connection would cause unexpected voltage drop of signals on the word lines. As a result, the memory chip would not function property.
It is difficult to detect such bridging effect in the manufacturing stage. An integrated circuit would undergo a number of tests for purposes of quality control before it is released in the market. Conventionally, these tests input signals into the integrated circuit and check whether the output signals are correct. The signals are usually at an ordinary voltage level and lasts in the integrated circuit for a short period of time. However, the metal migration is a long and gradual process that usually takes a long time for the migrate metal to fully develop as an electrical connection. An integrated circuit that passes the tests may still fail later on because of the undesired connection developed over a long period of operation. This would increase the product return rate by the customers and raises a serious reliability issue.
What is needed is a method and system capable of detecting potential defects caused by the bridging effect between adjacent lines in order to improve the reliability of an integrated circuit.